Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages including melt film formation, quenching and windup. For a general description of these and other processes associated with film-making, see K. R. Osborn and W. A. Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film at a desirable rate. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. Biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions. Biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces and tearing, leading to their greater utility in most packaging applications.
In flexible laminate film structures, an oriented film is adhered to a substrate film. While films may be readily bonded together through an adhesive, it is known that the initial bond strength degrades over a period of days. After used in packaging and shipped to consumers, the film may experience significantly reduced properties, including delamination. Thus, it would be highly desirable to have a film with acceptable initial bond strength that does not degrade as rapidly as in currently available film laminates.